Many optical systems require the use of a series of wavelengths. These different wavelengths are generally referred to as “channels.” For example, Dense Wavelength Division and Multiplexing (DWDM) communication systems exploit numerous different wavelengths in order to increase the throughput of the communication system. Other such systems include Differential Absorption Lidar (DIAL) systems, which are used for monitoring pollutants or small quantities of gases in the air. In these systems, the measurement is performed by transmitting beams having a multitude of closely spaced wavelengths, and afterwards detecting the backscattered beams. Generally, one of the beams, having a specific wavelength, is absorbed by a specific substance on the optical track, and the amount of absorption is measured by the ratios of the amplitudes of the backscattered beams.
In general, each single wavelength is obtained from a single source, which is usually a laser source, and the number of required sources is the number of different wavelength channels. Both the central wavelength of each channel and the wavelength variations, are determined by the properties of a specific source. Thus, in order to prevent overlapping of two adjacent wavelength channels, the spacing between these channels must be larger than the wavelength variations or tolerance of each single channel. The wavelength variations result mainly from temperature changes, but are also susceptible to opto-mechanical instabilities and fabrication tolerances. Since the wavelength range of an optical system is generally limited, the wavelength variations in such systems limit the total number of possible channels.
When operating a system wherein each wavelength channel is generated by a different light source or when there is a need in backup sources, an identical light source should be available in stock, which is costly. Alternatively, all of the channels could operate with a similar light source which has a tunable wavelength in a certain range and is fixed to a different wavelength for each channel. Here again, the tunability significantly increases the cost of the light source.
Some systems, in which one fiber laser source provided several wavelength channels with equal spacing between them, have been investigated in the past. However, in such fiber lasers, a single output beam is produced which consists of a multitude of wavelengths. Thus, the different wavelength channels are not separated either spatially or angularly and cannot be separately modulated.
In the optical receiver of multi-wavelength communication systems, it is generally required to split the incoming signal (composed of a multitude of wavelengths), into a multitude of channels, each having a single wavelength. This process is referred to as “optical de-multiplexing.” Several methods are widely used for de-multiplexing. These include exploitation of diffraction gratings, either inside optical fibers (known as “fiber Bragg gratings”), in a waveguide or in free space, the exploitation of prisms, the exploitation of interferometers, or other spectral filters.